A diaphragm is a mechanical element interposed on the path of a light beam in an optical system for defining the amount of light transmitted and the aperture of the system.
Such a device is especially used in high-performance imaging systems as it can ensure control functions of the light flow or regulation of the depth of field. It can also block diffracted rays in the optical system and minimize aberrations of the optical system.
The iris diaphragm, described in document US 21470 [1], is still widely used in recent and evolved optical systems. It comprises an assembly of mobile blades of variable number depending on lens size. A mechanism turns the blades and therefore regulates the aperture.
Several limitations are associated with this type of diaphragm.
First of all, this is a complex and expensive solution. The complexity of the mechanical structure (mechanism for displacement of blades) creates assembly difficulties. Also, a sufficient number of mobile blades has to be integrated to obtain a quasi-circular aperture (generally necessary for an optical system). The manufacturing cost of such a diaphragm is high and this technological solution therefore proves expensive.
Also, the power consumed by such a device is high. In effect, the force required for changing the aperture is impacted by the friction between the mobile mechanical pieces. It is therefore necessary to use powerful motors to modify the aperture.
In addition, the complex mechanical structure and the motors used provide a particularly bulky device.
Finally, wear of the mobile mechanical pieces limits the reliability of the diaphragm over time.
Taking the same approach as the iris diaphragm, novel mechanical solutions have been developed in recent years [2, 3].
Of these solutions mechanical, some are based on actuation by MEMS (micro-electromechanical systems) to optimize size and reduce consumption. Such is the case in references [2, 3] mentioned above.
However, these solutions fail to overcome all limitations of the iris diaphragm. In effect, these mechanical diaphragm technologies all have large dimensions and manufacturing complexity mainly connected with the same operating principle. Also, the majority of them do not produce a quasi-circular aperture required for optical systems. For several years now, novel non-mechanical solutions have been developed.
In particular, several fluidic solutions have been developed as an alternative to mechanical solutions. State of the art of diaphragms with variable aperture with fluidic structure is detailed in the thesis by Philipp Müller [4], a brief synthesis of which is presented herein below in reference to FIGS. 12A to 12G. Each of these figures represents the same respective device in two aperture configurations.
In the technological solution presented in FIG. 12A, the optical device 200 comprises a plurality of semi-spheres 201 made of transparent elastomer pressed against a substrate 202 made of PMMA and encapsulating an opaque liquid 203. The pressure and amount of opaque liquid located between the semi-spheres 201 and the substrate 202 are adjusted so as to more or less let light through and adjust the aperture diameter.
For this purpose, the device comprises an inlet 204 of opaque liquid coupled to a system such as a pump (not shown) external to the device 200. This device is particularly significant in creating an array of diaphragms. However, such a device is not integrated, as the system for pressurizing the opaque liquid is placed to the exterior of the device, such that this solution is bulky.
The device 300 illustrated in FIG. 12B comprises a deformable membrane 301 and a constant volume of opaque liquid 302 contained in a first cavity defined in part by the membrane 301. On the face opposite the opaque liquid 302, the membrane 301 is in contact with gas 303, for example air, contained in a second cavity. Said second cavity comprises a gas inlet 304 coupled to an external system (not illustrated) for pressurizing said gas. More or less substantial pressure is applied to the membrane 301 by the gas 303 introduced to or withdrawn from the cavity via the inlet 304. The opaque liquid 302 is pushed by the membrane 301, varying the aperture of the diaphragm. This produces the same disadvantage as for the previous solution (non-integrated solution) due to the pressurizing system being external.
The device 400 illustrated in FIG. 12C comprises an opaque liquid 401 trapped between a glass substrate 402 and a deformable membrane 403 actuated by an annular piezoelectric actuator arranged at the periphery of the membrane. Under the effect of the actuator, the opaque liquid 401 is pushed from the center of the device and the center of the membrane is pressed progressively on the substrate 402. This solution is integrated, the opaque liquid 401 being encapsulated at constant volume, without it being necessary to provide an inlet/outlet for liquid or an outer pressurizing system). However, the dimension is still considerable (of the order of 25 mm per side). A major disadvantage to this solution is the resulting mediocre optical quality. In effect, when the membrane 403 is pressed against the substrate 402, a small amount of opaque liquid can remain locally which compromises the optical quality of the ensemble. Also, the solid/solid interface between the membrane and the glass generally produces a large error on the wavefront and mediocre optical quality of the bandwidth of the diaphragm. Once the membrane 403 and the substrate 402 are in contact, adhesion between the two respective materials can complicate or even prevent reverse operation and return of the opaque liquid over all or part of this area.
The device 500 illustrated in FIG. 12D comprises a plurality of concentric micro-channels 501 and an intake 502 for opaque liquid 503. Similar to the devices of FIGS. 12A and 12B, this solution is not integrated.
The device 600 illustrated in FIG. 12E comprises an opaque liquid 601 and a liquid 602 transparent to the light beam to be transmitted, as well as two inlets (601a, 602a)/outlets (601b, 602b) for each of said liquids. This system is highly complex and achieves small aperture variations only. Also, the volume of liquid in the device is not constant and systems external to the device are necessary for operation to ensure the laminar flow of both liquids.
In the examples illustrated in FIGS. 12F and 12G, the device 700, respectively 800 comprises two liquids, one opaque and the other transparent to the light beam to be transmitted, and electrodes for adjusting the wettability of one of said liquids. The operating principle in these two cases is based on electro-wetting, a technique well known in the field of fluidics. In the case of FIG. 12F, a transparent electrode 701 made of ITO is sufficient to vary the wettability of the opaque liquid 703 (and therefore its radius of curvature) relative to a hydrophobic dielectric material 702 and to more or less open the central area of the device (the transparent liquid being designated by the marker 704). In the case of FIG. 12G, the principle is the same but the sole electrode is replaced by several interdigitated electrodes 801 to best control the form of the interface between the respectively transparent 803 and opaque 804 liquids (the hydrophobic dielectric being designated by the marker 802). In both these cases, the solution is integrated (liquids are encapsulated at constant volume, no need of inlet/outlet nor complementary outer system). The disadvantages relative to both these solutions are the significant thickness of the device (typically 2 mm) and the strong electrical power supply required (typically 100 V). This latter characteristic the makes the use and control of the device complex and impacts significantly the cost of the solution.